The present invention relates to a surface plasmon laser structure and, more particularly, to a surface plasmon laser including a distributed feedback (DFB) structure to provide long wavelength, single mode operation.
Existing technologies for long wavelength injection lasers based on interband transitions in III-V semiconductor materials are typically limited to xcex less than 5 xcexcm, leaving a considerable part of the mid- to far-infrared spectrum accessible only by lead-salt lasers.
However. quantum cascade (QC) lasers operating on intersubband transitions between conduction band states in InGaAs/AlInAs heterostructures have proven so far to be extremely versatile, covering the range of wavelengths of the two atmospheric windows (3.4-13 xcexcm), and providing high optical power at room temperature. When formed as a distributed feedback (DFB) device (including a grating structure embedded in the optical waveguide. in the immediate vicinity of the active region), single mode operation is possible. Optical waveguiding is achieved by virtue of the inner core (active) region having a higher refractive index than the surrounding (outer) cladding region. At longer wavelengths, however, the total thickness of the waveguide layers (core plus claddings) becomes difficult to handle and, moreover, light absorption by free carriers (particularly in the relatively high doped n-type QC cladding layers) results in even greater signal losses. Additionally, DFB structures operating at longer wavelengths require an extremely deep etch to form the grating structure, making regrowth problematic and leaving weakly coupled gratings far away from the active region as the only option. All of these difficulties result in the DFB structure being an unattractive candidate for long wavelength applications.
However, Maxwell""s laws of electromagnetism allow for another type of optical confinement to take place at the interface between two different homogeneous materials. These light waves exist, characterized by an exponentially decaying intensity in the two directions normal to the interface, provided the dielectric constants (∈) of the two materials have real parts of opposite sign. For a given radiation frequency, one single confined mode results, with the magnetic field polarized parallel to the interface and normal to the propagation direction (i.e;, transverse magnetic (TM) polarization).
Dielectric constants having a negative real part typically appear in the electromagnetic response of charged harmonic oscillators, more specifically, at frequencies above the oscillator resonance xcfx890 and up to a frequency xcfx89L, where ∈(xcfx89L)=0 and purely longitudinal modes can propagate. In metals or high-doped semiconductors. the existing nearly free electrons behave as simple oscillators of exactly zero resonance frequency for transverse excitation, while at the same time displaying very high xcfx89L, generally in the visible or even in the UV wavelength range. The latter takes the name of xe2x80x9cplasma frequencyxe2x80x9d xcfx89p, being the frequency of charge density oscillations. Thus, metals present an extremely wide range of wavelengths where Re[∈] less than 0, and where the metals can support the interface-confined electromagnetic waves, the waves referred to as xe2x80x9csurface plasmonsxe2x80x9d. The possibility of using surface plasmons in place of more conventional multi-layer dielectric waveguides at optical frequencies has been recently explored in the field of mid-infrared semiconductor lasers. However, the marginal performances of these surface plasmon devices cannot compete with those of traditional layered structures.
There remains a need in the art, therefore, for a relatively long wavelength laser (i.e., xcex greater than 15 xcexcm) that is not prohibitively thick, nor as technically difficult to manufacture as DFB devices.
The need remaining in the art is addressed by the present invention, which relates to a surface plasmon laser structure and, more particularly, to a surface plasmon laser including a distributed feedback (DFB) structure for providing single mode, long wavelength (e.g., xcex=17 xcexcm) emission.
A surface plasmon laser includes an active region formed as an insulated ridge structure and further comprises a metal surface layer disposed longitudinally along the ridge, contiguous with the active region. The structure results in the formation of surface plasmon propagation, where at wavelengths greater than 15 xcexcm it has been found that the power loss associated with penetration depth (i.e., skin depth) into the metal is largely reduced. The resultant large mode confinement xcex93 with the attendant reduced thickness of the waveguide layers (reduced from a prior art thickness of approximately 9 xcexcm to less than 4 xcexcm) is advantageously used to create a long wavelength laser.
In accordance with the teachings of the present invention, the metal surface layer comprises a metallic grating (i.e.; periodic) surface structure, thus forming a DFB surface plasmon laser capable of single mode emission. In one embodiment, titanium stripes are first deposited on the exposed surface of the active region, followed by a continuous layer of gold. The resulting Ti/Auxe2x80x94Au grating provides single mode plasmon radiation output, where the output wavelength can be xe2x80x9ctunedxe2x80x9d by modifying the operating temperature of the device.
In a particular embodiment of the present invention, the active region of the DFB surface plasmon laser may comprise a quantum cascade (QC) structure, including a multiplicity of essentially identical repeat units, each repeat unit comprising one or more quantum wells. Successive carrier transitions from a higher to a lower energy state result in photon emissions, with the photon energy depending on the structure and compositional details of the repeat unit.
Other and further advantages and arrangements of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.